Definition
In patent law, double use refers to the application of a previously known device, process, or mechanism to a new purpose that is not sufficiently distinct from its original purpose to qualify as a patentable invention. A new application of an old device is characterized as mere double use when the new application would readily occur to a person of ordinary mechanical skill — that is, when no genuine inventive step separates the old use from the new one. Because patent law rewards inventive ingenuity, not the mechanical transfer of known devices to analogous tasks, a double use does not support a valid patent claim.
The doctrine functions as a limit on patentability: the novelty of a use, standing alone, is insufficient to secure patent protection if the new use is so nearly analogous to the prior use that ordinary skill in the relevant trade would bridge the gap without creative effort.
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Common Language
Modern common usage (Wiktionary): "Double use" in ordinary language suggests using something for two purposes simultaneously, or using an item in a secondary capacity beyond its primary function.
Historical common usage (Webster's 1913): No specific entry for "double use" as a fixed phrase; the component words carry their ordinary meanings of duplication and application.
The legal meaning diverges sharply from both common senses. Legally, the term does not describe an item serving two functions at once. It describes a single secondary application of an old device that fails to rise to the level of invention — the "double" refers to the repetition of an existing use concept in a new context, not simultaneous dual function.
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Common Confusion
Double use is sometimes confused with the broader obviousness inquiry in patent law. While related, they are not identical. Obviousness is the modern statutory standard under which a claimed invention is evaluated against the prior art as a whole. Double use is a narrower, older doctrinal label specifically addressing the scenario where a device is transplanted from one analogous application to another. A double use finding is, in effect, an application of the obviousness principle to a particular factual pattern, but historical sources treat it as a distinct term of art. Researchers should not assume that every obviousness rejection in historical patent records will use the phrase "double use," nor that every double use characterization maps cleanly onto modern obviousness doctrine.
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Why It Matters in Research
Double use is primarily a term of nineteenth and early twentieth century patent law discourse. Researchers working in historical patent records, patent treatises, or early appellate decisions will encounter it as a functional substitute for what modern patent law handles under the obviousness standard codified in federal statute. The term largely disappeared from active legal vocabulary as modern patent doctrine consolidated around the "nonobvious subject matter" requirement.
Several navigational traps apply. First, the phrase appears almost exclusively in older sources; searching for it in post-mid-twentieth century materials will yield sparse results. Second, the threshold inquiry embedded in the term — whether the new application would occur to a person of ordinary mechanical skill — is the direct ancestor of the "person having ordinary skill in the art" (PHOSITA) standard central to modern obviousness analysis. Researchers tracing the genealogy of that standard should treat double use doctrine as a significant antecedent. Third, double use should not be confused with "double patenting," a distinct doctrine prohibiting a patentee from obtaining two patents covering the same invention or an obvious variation thereof. The two doctrines address related but separate problems and appear in different procedural contexts.
For corpus researchers, Bouvier's entry on double use is the primary historical dictionary touchstone. The entry is brief and incomplete as reproduced, but its core formulation — that the new use must go beyond what would occur to a person of ordinary mechanical skill — captures the essential test.
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Historical Dictionary Support
Bouvier's Law Dictionary provides the only historical legal dictionary treatment located in the Law Mind corpus. Bouvier defines double use as a term in patent law signaling that a later device is merely a new application of an older device, not involving the exercise of the inventive faculty. The standard Bouvier articulates — whether the new use is so nearly analogous to the former one that it would occur to a person of ordinary mechanical skill — closely tracks the language of nineteenth-century patent case law.
No entry for double use appears in Black's Law Dictionary in the corpus sources, reflecting the term's gradual displacement by the language of obviousness in the twentieth century. Researchers should treat Bouvier as the authoritative historical reference for this term and supplement it with period treatises on patent law when greater doctrinal depth is needed.
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Jurisdictional Note
Double use as a doctrine operated within federal patent law, which is exclusively federal in the United States. Jurisdictional variation is not a significant research concern for this term; the relevant variations are temporal, not geographic, tracking the evolution from double use doctrine toward modern statutory obviousness analysis.
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Encyclopedia Cross-Reference
The Law Mind Intellectual Property Encyclopedia, ip_69: Trademark Use in Commerce and the Use Requirement — tangentially relevant; addresses the role of "use" as a threshold concept in IP law, though in the trademark rather than patent context. No encyclopedia entry directly addresses patent obviousness or double use doctrine in the current corpus.
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